In a vehicle mounting an automatic transmission that comprises a fluid transmission, the vehicle has a structure that a hydraulic pressure is produced by a hydraulic pump driven by an engine, and a multiple-disc clutch is engaged by the hydraulic pressure to secure a gear position. When the engine is brought into a halt, therefore, no hydraulic pressure can be produced and consequently, the transmission automatically becomes a neutral state at the time of shutdown of the engine. Therefore, the vehicle mounting the automatic transmission that comprises a fluid transmission is not capable of limiting the rotation of the wheels by utilizing the frictional force of the engine. Accordingly, the vehicle mounting the automatic transmission that is a fluid transmission has a parking gear that is provided on the output shaft of the transmission and is equipped with a parking lock mechanism, which selectively engages with the above parking gear, so that the parking lock mechanism is engaged with the parking gear at the time of parking. When the parking gear provided on the output shaft of the transmission is locked, however, a maximum output torque of the transmission acts on the parking lock mechanism creating a condition, which is very disadvantageous from the standpoint of strength. In the medium-to heavy-sized trucks mounting the automatic transmissions, therefore, the parking lock mechanism is not established from the standpoint of strength and hence, the parking lock mechanism is hardly furnished.
Meanwhile, in the vehicles using a parallel shaft gear type transmission and using a hydraulic multiple-disc clutch or an electromagnetic powder clutch, too, the clutch is disconnected when the engine is brought into a halt and hence, the rotation of the wheels cannot be limited by utilizing the frictional force of the engine. In the vehicles of this type, too, therefore, it can be contrived to provide the output shaft of the transmission with a parking gear and to lock the parking gear at the time of parking. In the medium-to heavy-sized trucks, however, the parking lock mechanism is not established from the standpoint of strength as described above. To solve this problem, JP-A 2000-264178 discloses a parking device for a vehicle having a parking gear provided on the input shaft of the transmission so that the parking gear is locked at the time of parking.
However, in the parking device for vehicles in which the input shaft of the transmission is provided with a parking gear so that is locked at the time of parking, as is described in the above publication, the transmission must be brought into engagement with a predetermined gear position prior to locking the parking gear. When the change lever is operated to the parking position while the vehicle is traveling, however, it is operated to the parking position via the neutral position. Therefore, the clutch is disconnected and the transmission remains in the neutral state. Namely, in the device in which the parking lock mechanism is mechanically coupled to the change lever, no driving force acts on the input shaft of the transmission and hence, the parking lock mechanism works if the change lever is shifted to the parking position. As a result, the input shaft is locked making it difficult to bring the transmission into engagement with a predetermined gear position. Even when the change lever is operated to the parking position at the time of parking, therefore, the transmission remains in the neutral state arousing a problem in that the parking lock function does not work.